A number of gaps exist in our understaning of muscle contraction. One of these is the role of proteins other than myosin in organization of the thick filament. A second is the mechanism of control of the rate of formation and disruption of actin-myosin crossbridges. The smooth and striated adductor muscles of the clam appear to be exceptionally favorable muscles for clarification of these two points. These muscles contain in large amounts a protein paramyosin that is readily isolated for study and which appears to be involved in both of the functions mentioned above. Paramyosin occurs as the core of the thick filaments upon which myosin is deposited, therefore, it has a structural role in organization of the thick filament. In addition, it appears to be involved in the interaction of myosin of the thick filament with actin of thin filaments. Hence, it may have a second role in regulation of the rate of muscle contraction-relaxation processes. The initial objective will be to establish the capacity of paramyosin to bind myosin; determine the specific regions on both myosin and paramyosin essential for binding; and determine the conditions that affect the tightness of binding. The degree of binding will be measured by fluorescence and light-scatter techniques. A second objective will be to assess effects on myosin binding by conformational change in the unstable C-terminal region of paramyosin. These conformational changes will be studied by circular dichroism, infra-red spectroscopy and hydrogen-tritium exchange. A third objective will be to determine the nature of the inhibitory effect of paramyosin on actomyosin ATPase activity.